Printed products which consist of a plurality of folded sheets are e.g. produced in intaglio printing whereby the printed paper web is cut lengthwise, the cut web parts are guided over each other, the superimposed web parts are cut transversely to form groups of superimposed sheets and the groups of superimposed sheets are folded transversely. The groups of sheets such folded inside each other (tabloids) are then normally stitched along the folding line or connected to each other with other suitable methods and are then, if required, trimmed along up to three edges.
Voluminous printed products are normally produced from a plurality of product parts as described above, whereby each product part has a smaller number of sheets. Several such product parts are brought together by collecting or inserting after folding or by collating before or after folding and are then stitched or glued (and, if required, trimmed) to form a finished product. The production via product parts allows a larger printing capacity compared to the direct production of voluminous products and facilitates the addition of additional products (stuck-in cards, sample bags etc.) and/or the application of additional printing (individualised information such as addresses etc.) on inside pages of the finished product.
However, the production via product parts also has the disadvantage that more steps are necessary, especially steps in which the groups of loosely superimposed or loosely folded sheets (product parts) must be manipulated. During these steps the product parts must be guided carefully or the sheets in the groups must be repositioned again and again, especially if the sheets consist of paper which slides easily.
Furthermore, for collecting or inserting after folding at least a part of the groups of sheets loosely folded inside each other must be opened exactly in the middle. For this purpose the sheets are usually folded off-center which facilitates the opening considerably or makes it possible at all. However, the protruding part of the sheets produced by the off-center folding must be trimmed off the finished product which increases the amount of waste considerably.
In order to facilitate manipulation and opening of the mentioned product parts consisting of sheets loosely folded inside each other, it is e.g. suggested in the publication DE-2910964 to punch a line of holes along at least one edge running perpendicular to the fold line or to slit the edges and deform them in order to create more adhesion between the sheets. An edge area treated thus, similarly to off-center folding, regards every single sheet and must be trimmed off the finished product as it is not tolerable on the finished product for aesthetic reasons.
A further known method for stabilizing the product parts which consist of loosely superimposed or loosely folded sheets is an electrostatic charging of the paper such that the paper layers adhere to each other. This is advantageous for stabilizing the layers but it may create problems in subsequent processing steps, e.g. when the product parts must be opened.